1. Field of the Invention
The present invention relates to a plasma display panel (PDP) having a protection film for protecting a dielectric layer of the PDP from discharge and a fabrication method of the same plasma display panel and, particularly, the present invention relates to a plasma display panel having a magnesium oxide (MgO) protection film for improving discharge characteristics of the plasma display panel and a fabrication method of the same.
2. Description of the Prior Art
In general, the plasma display panel (PDP) is featured by thin structure, flicker-free display, high display contrast ratio, possibility of providing a relatively large display screen, short response time and self-light emission type with which a multi-color display can be realized by using various phosphor materials, etc. Therefore, the PDP is becoming popular in the field of color image display, which is related to computers.
The PDP is classified to an AC type PDP and a DC type PDP. In the AC type PDP, a protection film for preventing a dielectric layer formed in cells of the PDP from being damaged by discharge is provided. Requirements of a material of the protection film of the PDP for protecting the dielectric layer thereof from electric discharge are (1) high durability against ion bombardment, (2) high secondary electron emission coefficient and (3) high insulating characteristics. The protection film is generally formed of magnesium oxide (MgO), which satisfies those requirements. The MgO protection film is generally vapor-deposited on a PDP substrate by heating and sublimating MgO particles as a material by using electron beam (EB) vapor-deposition or formed by MgO ion plating (IP).
JP 2000-63171A and JP H10-291854A disclose fabrication methods of a MgO protection film of a PDP, in which impurity metal ion density of a MgO material is lowered to lower firing voltage of the PDP.
Although the definition of the firing voltage is not described in JP 2000-63171A, JP H10-291854A discloses the evaluation method of the discharge start voltage (cf. paragraph 0037 of the specification thereof and FIG. 2 of the drawings thereof). According to the evaluation method disclosed therein, the firing voltage of the PDP is monitored by increasing a voltage applied between surface discharge electrodes of the PDP.
However, the inventors of the present invention have found that, according to the evaluation method using the firing voltage as described in the above mentioned prior arts, an evaluation result obtained in a case where the evaluation is performed after the aging of the PDP, that is, in a constant cycle from sustaining discharge through priming to write discharge, that is, under conditions of the practical use of the PDP and an evaluation result obtained according to the disclosed prior art methods, which use the mere comparison of a firing voltage becomes different. That is, the present inventors have found that, according to the prior art evaluation method, the difference of firing voltage due to difference in impurity density between the deposition materials can not be found when the aging time of the PDP exceeds 20 hours even if there is a difference in priming voltage between the materials.
Therefore, the conventional evaluation method of the MgO protection film is meaningless in evaluating the priming voltage of the practical PDP. Accordingly, it is impossible to obtain a MgO protection film having a low priming voltage by merely defining the metal ion density of the deposition material of the MgO protection film.
Furthermore, although, in evaluating the drive characteristics of a PDP, it is necessary to once reset a sustaining discharge of the whole area of the PDP by a priming discharge, to lighten the whole area and then to darken the whole area by a priming erase, it is difficult to uniformly reset the sustaining discharge by a low priming voltage since the priming voltage of the conventional protection film is high. If the uniform resetting of the sustaining discharge is impossible, there may be erroneous lightening or flicker. Although it is possible to uniformly reset the sustaining discharge by increasing the priming voltage, high priming voltage may produce large discharge, resulting in that the luminance due to priming, that is, black luminance, is increased and contrast is lowered.
It has generally known that the impurity metal ion density of the MgO protection film formed by the electron beam vapor deposition is increased compared with the metal ion density of the MgO deposition material of which the MgO protection film is formed. However, the present inventors have found that it is possible to restrict the increase of the impurity metal ion density of the MgO protection film by forming the MgO protection film of the MgO material in a hydrogen ion environment.
Incidentally, in order to improve the orientation of the MgO film, JP H9-295894A discloses a method for forming a MgO film in an environment containing exited or ionized hydrogen atoms. However, in the film forming method disclosed therein, the orientation plane is not constant although the orientation itself is improved. Therefore, there may be cases where the sputtering durability characteristics of the MgO protection film becomes insufficient. Moreover, the crystal grain size of the MgO film becomes smaller and the firing voltage becomes higher.